I-III-VI2 compounds of the CuIn(1-x)GaxSeyS(2-y) type (where x is substantially between 0 and 1 and y is substantially between 0 and 2) are regarded as very promising and could constitute the next generation of thin-film photovoltaic cells. These compounds have a wide direct bandgap of between 1.05 and 1.6 eV, which allows solar radiation in the visible to be strongly absorbed.
Record photovoltaic conversion efficiencies have been achieved by preparing thin films by evaporation on small areas. However, evaporation is difficult to adapt to the industrial scale because of problems of nonuniformity and low utilization of raw materials. Sputtering is better suited to large areas, but it requires very expensive vacuum equipment and precursor targets.
There is therefore a real need for alternative, low-cost atmospheric-pressure, techniques. The technique of thin-film deposition by electrochemistry, in particular by electrolysis, appears to be a very attractive alternative. The advantages of this deposition technique are numerous, and in particular the following:                deposition at ambient temperature and ambient pressure in an electrolysis bath;        possibility of handling large areas with high uniformity;        ease of implementation;        low installation and raw material costs (no special forming operation, high level of material utilization); and        great variety of possible deposit shapes due to the localized nature of the deposit on the substrate.        
Despite extensive research in this field, the difficulties encountered relate to how to control the quality of the electrodeposited precursors (composition and morphology) and, more particularly, the difficulty of inserting metals such as gallium or aluminum (elements III) whose electrodeposition potential is very cathodic.
I-III-VI2 compounds in which:                the element I corresponds to Cu;        the element III corresponds to In and to Ga and/or Al; and        the element VI corresponds to Se and/or S, will be denoted hereafter by the abbreviation CIGS.        
Moreover, the term “film” is understood to mean a thin layer deposited on a substrate, and the term “precursor film” is understood to mean a thin layer of overall composition close to I-III-VI2 and obtained directly after deposition by electrolysis, with no optional subsequent treatment.
As regards pure electrodeposition of CIGS (with no evaporation step), the morphology and the composition of the precursors are very difficult to control, as the following documents indicate:                “One step electrodeposited CuIn1-xGaxSe2 thin films: structure and morphology”, M. Fahourme, F. Chraibi, M. Aggour, J. L. Delplancke, A. Ennaoui and M. Lux-Steiner, 17th European Photovoltaic Solar Energy Conference, Oct. 22-26, 2001, Munich, Germany; and        “CuIn1-xGaxSe2-based photovoltaic cells from electrodeposited precursor films”, Materials Research Society Symposium—Proceedings, Volume 668, 2001, pages H8101-H8106, by R. N. Bhattacharya and Arturo M. Fernandes.        
The most recent developments have involved an evaporation step after the electrodeposition, so as to increase the In and Ga contents of the electrodeposited films. In these developments, especially those described in document WO 01/78154, the electrodeposition is an actual codeposition of the elements Cu, In, Ga and Se (in order to obtain a quaternary alloy) and employs a method of deposition in a pH buffered electrolytic bath. The buffer solution is composed of sulfamic acid and potassium biphthalate, forming a buffer of the pHydrion (registered trademark) type. Electrodeposited films that have given photovoltaic cells using the hybrid method involving an electrodeposition step followed by an evaporation step have a dendritic morphology of low density.